As I understand you already have a SnapMirror replication configured from Cluster A to Cluster B, you now wish to configure an additional chain for the existing replication (aka cascade replication) to a third cluster (Cluster-C).
Once the new chain has been initialized you will have to setup schedules in order to copy the data across each of the links of the cascade at a defined frequency and interval of your choice.
Short answer is yes they have to be configured seperately.
Thanks